Our overall objective is to investigate the efferent and higher order afferent projections from individual vestibular receptors in the adult pigeon and adult monkey. More specifically, we will (1) demonstrate vestibular efferent projections from individual cristae and maculae in the adult animal using horseradish peroxidase (HRP) reacted by the highly sensitive tetramethylbenzidine blue reaction process, (2) demonstrate higher order afferent projections from individual vestibular receptors using transsynaptically transported 3H-labeled proline-fucose, (3) determine if the HRP-labeled efferent neurons in the reticular formation are catecholaminergic with fluorescence techniques, and (4) investigate whether an HRP-labeled reticular efferent neuron projects to both labyrinths simultaneously with electron microscopy. To do this, we will (1) deliver discrete amounts of neural tracer equally, and without damage, to the entire receptor neuroepithelial surface by injecting and confining HRP or proline-fucose to the membranous endolymphatic space, (2) isolate individual vestibular receptors by intracranial selective vestibulocochlear nerve axotomy thereby permitting intraaxonal tracer transport from only the isolated receptor neuroepithelium, and (3) use transcardiac bilateral intracarotid catheterization to obtain ultrastructural quality fixation of the brain, labyrinths, and upper spinal cord. We will construct accurate 2-plane (coronal, sagittal) neuronal maps (with cell counts, cell dimensions, and fiber tract densities) of labeled efferent and higher order afferent projections within the brain and spinal cord and synthesize our findings to define further the anatomical organization of individual vestibular receptor projections. The expected results will provide a sound basis for future electorphysiological studies, and indirectly contribute to a better understanding of clinical disorders such as Meniere's disease.